1. Field of the Invention
The present invention relates to a carbon electrode material used for a vanadium-based redox-flow type battery and, more particularly to, such a carbon electrode material which is excellent in the energy efficiency of the overall battery system and also which has less changes in its performance accompanied by a prolonged period of services.
2. Description of the Related Art
Conventionally, the electrode has been developed intensively as a key factor of the battery performance. Some types of electrodes do not act as an active material but act as a reaction field which promotes the electrochemical reaction of active materials, often coming in of carbon materials due to their good electro-conductivity and chemical resistance. In particular, as redox-flow type battery electrodes which have been developed popularly for power storing purposes, a carbon fiber assembly having a chemical resistance, electro-conductivity, and liquid flowability is us ed.
The redox-flow type battery has been changed from a type using an aqueous solution of iron and hydrochloric acid in its positive electrode and an aqueous solution of chromium and hydrochloric acid in negative to a type using in both electrodes an aqueous solution of vanadium sulfate having a higher electromotive force and thus improved to have a higher energy density. It is recently having a further enhanced energy density by the development for a higher concentration of active materials employed therein.
The redox-flow type battery mainly comprises, as shown in FIG. 1, external tanks 6 and 7 for storing an electrolyte solution and an electrolytic cell EC, in such a configuration that while pumps 8 and 9 are sending the electrolyte solution containing an active material from the external tanks 6 and 7 to the electrolytic cell EC, electrodes incorporated in the electrolytic cell EC performs electrochemical conversion, i.e. charge-discharge.
In charge-discharge in general, the electrolytic cell has such a liquid flow-through type structure as shown in FIG. 1, in order to circulate an electrolyte solution between the external tanks and the electrolytic cell itself. Such a liquid flow-through type electrolytic cell is called a single cell and used, as a minimum unit, independently or in a multi-layer stack. Since the electrochemical reaction within a liquid flow-through type electrolytic cell occurs as a non-uniform phase reaction on the surface of an electrode, it is generally accompanied by a two-dimensional electrolytic-reaction field. Since its electrolytic-reaction field is a two-dimensional one, the electrolytic cell suffers a problem that it has a smaller reaction amount per unit volume.
Therefore, the electrochemical reaction field has been three-dimentionalized in order to increase a reaction amount, i.e. current density per unit area. FIG. 2 shows an exploded perspective view of a liquid flow-through type electrolytic cell having three-dimensional electrodes. In this electrolytic cell, between counter-opposing two collector plates 1 an ion exchange membrane 3 is interposed, flow passages 4a and 4b for an electrolyte solution along the inner surface of the collector plates 1 are formed by a spacer 2. At least one of these flow passages 4a and 4b has therein an electrode material 5 such as a carbon fiber assembly, thus constituting a three-dimensional electrode. Note here that the collector plate 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolyte solution.
In the case of a redox-flow type battery using vanadium oxysulfate as its positive-electrode electrolyte solution and vanadium sulfate as its negative-electrode electrolyte solution, at the time of discharge, the solution containing V2+ is supplied to the liquid flow passage 4a at the negative electrode and the electrolyte solution containing V5+ (actually an ion containing oxygen) is supplied to the flow passage 4b at the positive electrode. At the flow passage 4a of the negative electrode, in the three-dimensional electrode 5, V2+ releases electrons, to be oxidized into V3+. These released electrons pass through an external circuit and reduces, within the three-dimensional electrode of the positive electrode, V5+ into V4+ (actually ions containing oxygen). As this oxidation-reduction reaction goes on, the electrolyte solution at the negative electrode runs short of SO42xe2x88x92 and that at the positive electrode has an excessive amount of SO42xe2x88x92, so that SO42xe2x88x92 moves from the positive electrode through the ion-exchange membrane to the negative electrode, thus maintaining a charge balance. Or H+ moves from the positive electrode through the ion-exchange membrane to the negative electrode, thus maintaining the charge balance. At the time of charge, the reaction opposite to that for discharge proceeds.
The electrode materials for vanadium-based redox-flow type batteries must have the following performance as their properties:
1) No side reactions occurring other than desired reactions (high reaction selectivity), specifically, high current efficiency (xcex7I).
2) High activity, specifically, small cell resistance. That is, high voltage efficiency (xcex7v).
3) High battery energy efficiency (xcex7E) related to the above-mentioned items 1) and 2).
xcex7E=xcex7Ixc3x97xcex7v
4) Less deterioration against repetitive use (prolonged service life), specifically, less deterioration of battery energy efficiency (xcex7E).
Japanese Patent Publication No. Sho-60-232669, for example, suggests that a carbonaceous material should be used as an electrode material for ion-chromium-based redox-flow type batteries, which has an quasi-graphite microcrystal having an average of 3.70 xc3x85 or less of a  less than 002 greater than  inter-facial spacing obtained with X-ray wide-angle analysis and an average of 9.0 xc3x85 or more of a c-axial crystallite size and also that has a total acid functional group amount of at least 0.01 meq/g.
Japanese Patent Publication No. Hei-5-234612 also suggests that carbonaceous fiber made from polyacrylonitrile-based fiber as a material should be used as an electrode material for an electrolytic cell of iron-chromium-based redox-flow type batteries, which has an quasi-graphite crystal structure with a  less than 002 greater than  inter-facial spacing of 3.50-3.60 xc3x85 as obtained with X-ray wide-angle analysis, and in which the number of the combination oxygen atoms is 10-25% of that of the carbon atoms on the surface.
By Japanese Patent Publication Nos. Sho-60-232669 and Hei-5-234612, however, to effectuate proper wettability between the carbonaceous material surface and the electrolyte solution, the total acid functional group amount must be 0.01 meq/g or more or the number of the combination oxygen atoms must be 10% or more of that of the carbon atoms on the carbonaceous material surface. Therefore, as found out recently, a vanadium-based redox-flow type battery presently under development having a higher active material concentration and also a higher viscosity provides a higher contact resistance between the carbonaceous material surface and the collector plate or that between the fiber and the fiber, resulting in the cell resistance being too high to obtain a high energy efficiency.
Moreover, it was found out that since the vanadium-based redox-flow type battery has strong oxidization force of the penta-valent ions of vanadium, the above-mentioned electrode material cannot provide sufficient oxidization resistance, so that the cell resistance increases as the charge-discharge cycle is repeated, thus increasing a change in (deterioration ratio of) the energy efficiency.
In view of the above, it is an object of the present invention to provide such an electrode material for vanadium-based redox-flow type batteries that can enhance the total efficiency of those vanadium-based redox-flow type batteries and also that improve the charge-discharge cycle service life.
It is another object of the present invention to provide a carbon electrode material that can enhance the electro-conductivity of the carbon electrode material itself and also that can inhibit the deterioration of the electro-conductivity due to a prolonged period of service.
It is still another object of the present invention to provide a carbon electrode material that can reduce the contact resistance of the carbon electrode surface and also that can maintain at a low value of the contact resistance of the carbon electrode material for a prolonged period of time.
It is still another object of the present invention to provide a carbon electrode material that can improve both the physical properties of a carbon fiber and those of a non-woven fabric, to reduce the cell resistance of a redox-flow type battery or permit smooth flow-through of the electrolyte solution within the electrolytic cell, thus enhancing the energy efficiency.
To this end, the inventors of the present invention conducted intensive study and came up with a finding for completion of the present invention that for a carbon electrode material having a particular quasi-graphite crystal structure, by regulating both a surface acid functional group amount and a number of surface bound-nitrogen atoms within their respective particular ranges, the above-mentioned objects can be achieved.
That is, any carbon electrode material for vanadium-based redox-flow type batteries according to the present invention is such a carbon electrode material for a vanadium-based redox-flow type battery, comprising quasi-graphite crystal structure, wherein  less than 002 greater than  spacing is 3.43 to 3.60 xc3x85, size of a crystallite in c axial direction is 15 to 33 xc3x85, and size of a crystallite in a axial direction is 30 to 70 xc3x85, each obtained by X-ray wide angle analysis, and having surface property obtained by XPS surface analysis wherein an amount of surface acidic functional groups is 0.1 to 1.2% relative to total number of surface carbon atoms and number of surface bound-nitrogen atoms is 5% or smaller relative to total number of surface carbon atoms. Preferably, in particular, the above-mentioned surface acid functional group amount is 0.2-1.0% of the total number of the surface carbon atoms and the above-mentioned number of the surface bound-nitrogen atoms is 3% or less of the total number of the surface carbon atoms.
In such a carbon electrode material having the above-mentioned quasi-graphite crystal structure, by regulating the surface acid functional group amount within the above-mentioned range, the vanadium-based redox-flow type battery has therein a remarkably increased degree of wettability with an electrolyte solution, an increased area of the electrode""s effective area (significantly larger than the geometrical area but smaller than the BET surface area), a largely improved degree of the electrode activity, and a largely enhanced electro-conductivity of the electrode surface. This in turn decreases the cell resistance, that is, improves the voltage efficiency xcex7v and largely enhances the energy efficiency xcex7E. Also, with the number of the surface bound-nitrogen atoms held in the above-mentioned range, proliferation (growing gigantically) of the functional groups because of repetition of the charge-discharge cycle can be inhibited, to also prevent deterioration of the accompanied electro-conductivity and wettability, i.e. increases in the cell resistance. With this, it is possible to reduce time-wise changes in the energy efficiency xcex7E during a prolonged charge-discharge cycle.
In the above case, preferably that  less than 002 greater than  inter-facial spacing is 3.45-3.50 xc3x85, that c-axial crystallite size is 20-30 xc3x85, and that a-axial crystallite size is 45-65 xc3x85. A carbon electrode material having such an quasi-graphite crystal structure has its- electro-conductivity further improved and therefore enjoys more appropriate cell resistance and energy efficiency also with less fluctuations in energy efficiency which may be caused by repetition of the charge-discharge cycle.
Preferably a carbon electrode material according to the present invention is used in a vanadium-based redox-flow type battery having its electrolyte-solution viscosity of 0.005 Paxc2x7s or higher at 25xc2x0 C. The redox-flow type battery having such an electrolyte solution tends to have a higher contact resistance between the carbonaceous material surface and the collector plate or between the fiber and the fiber, so that the carbon electrode material according to the present invention having the above-mentioned function and effects is particularly useful.
Also, preferably, the carbon electrode material according to the present invention is used in a vanadium-based redox-flow type battery wherein its electrolyte solution contains 1.5 mol/l or more of vanadium ions. Since the redox-flow type battery having such an electrolyte solution has a strong oxidization force of the vanadium""s penta-valent ions, as the charge-discharge cycle is repeated, the cell resistance increases due to the oxidization of the electrode material, thus causing the energy efficiency to deteriorate. Therefore, the carbon electrode material having the above-mentioned function and effects according to the present invention is particularly useful.